Lithium-ion rechargeable batteries and nickel metal hydride rechargeable batteries are well-known electricity storage devices installed in electric vehicles (EVs) and plug-in hybrid vehicles (PHVs). Patent Document 1 discloses an example of an electricity storage device that includes an electrode assembly formed by layering electrode sheets. Each electrode sheet includes metal foil and active metal layers formed on surfaces of the metal foil. Separators are located between the electrode sheets.
In Patent Document 1, two separators sandwich a rectangular positive electrode sheet. The sections of the separators that protrude beyond the four edges of the positive electrode sheet in in-plane directions are heat welded (thermally fused) to form weld regions. In the electricity storage device of Patent Document 1, each positive electrode sheet is thus positioned relative to separators. In addition, positive electrode sheets, which are covered by separators, and negative electrode sheets are layered so that electrode sheets with different sizes are layered face-to-face.